On Wed, 2004-07-07 at 01:35, Jeff Garzik wrote:
> >
> > True returning 0 this is not wrong; it
> > results in an extra call in the layer above the driver.
> > (I was trying to point that out in earlier email)
>
> er, I'm confused now?
>
> Every single ethernet driver returns zero, when it has queued a packet
> to hardware :) That's the common case, I would hope it doesn't result
> in additional work.
Its a small optimization. In either case, the xmit() will not be invoked
again until device wakes up - so both models are fine from that
perspective. In the case of returning a zero there will be a few more
lines of code executed trying to see if qdisc has a packet before
realizing the device is stopped and requeueing that packet.
Not something to sweat over to be honest. If i had 1000 hours available
and nothing fun to work on (theres always something fun to do) then i
will start changing things. I think you should encourage new drivers to
use the return 1 scheme though.
> >
> > Ok, I overlooked fragments. I think it would be useful to capture this
> > in the doc you were preping. BTW, why can you figure out the fragment
> > count? If you can then the check for number of descriptors availabel
> > could account for that.
>
> In one design, you can say
>
> queue packet
> if (free descriptors < MAX_SKB_FRAGS)
> netif_stop_queue()
> return 0
>
> That design wastes descriptors, but ensures you always have enough room
> when ->hard_start_xmit is called, and thus ensures you never have to
> return 1.
>
> Another design, that attempts to use more descriptors, is
>
> if (free descriptors < skb->frags)
> netif_stop_queue()
> return 1
>
> queue packet
>
> if (free descriptors == 0)
> netif_stop_queue()
> return 0
>
Looking good.
cheers,
jamal